1. Field of the Invention
The present invention relates to an improvement of a vibration sensor for use in various technical fields.
2. Description of the Prior Art
Piezoelectric vibration sensors used for detecting vibration from various vibration sources are generally classified into two large types, resonant type and nonresonent type. In the former type, the resonant frequency of the sensor is set up to be virtually the same as the frequency of the object of measurement, and thus, the sensor is arranged to have a sensitivity characteristic such that it is highly sensitive only in a set frequency band. On the other hand, the latter type is arranged to have a flat sensitivity characteristic such that it is evenly sensitive over a wide range of frequency band.
As an introduction to an explanation of the present invention, the construction of a nonresonant vibration sensor utilizing a piezoelectric element will be described below. What is shown in FIG. 3 is a first example of a vibration sensor (knocking sensor) used for detecting vibration in an engine of an automobile. The vibration sensor 10 is constructed of a base member 11, in the form of a sleeve with a flange portion 11a provided at its lower part and a screw portion 11b formed at its upper part, a fixing hole 12 bored in the center of the base member 11 for fixing the sensor, for example, to an engine, a piezoelectric element 13, a mass member 14 about the periphery of the base member 11 a nut 16 for firmly holding the piezoelectric element 13 and the mass member 14 through a suitable washer 15, electrode portions 17 placed in contact with upper and lower sides of the piezoelectric element 13, respectively, insulating members 17a, and lead wires 18 connected to the electrode portions 17. A vibration sensor of the described structure is known as the structure of an accelerometer, and is described, for example, in U.S. Pat. No. 3,307,054.
The use of a resin 19 as a covering as shown in FIG. 3, is also known as a simple housing forming method. In the case of FIG. 3, the resin integrally covers the members 11 to 17 and a portion of the lead wires 18 and constitutes a case member. For example, a housing is provided by polyurethane resin 19.